Welding is a manufacturing or fabrication process that bonds materials, usually metals or thermoplastics, by causing coalescence—the process by which two separate units grow together, fuse, or merge into a single body. The materials are joined by liquefying or plasticizing (e.g., soften without liquefying) the areas to be bonded together, generally through the application of heat and/or pressure over time, promote coalescence of the liquefied or plasticized material, and allow the coalesced material to cool, thereby completing the bond. Welding can be used, for example, to join two or more workpieces or for operating on a single workpiece (i.e., to repair a crack or join a member).